258 STUDY OF PIG EMBRYOS. 



deck-plate, the upper or dorsal limits of the dorsal zones, D.Z, are brought 

 far apart and the cavity of the hind-brain is thus enlarged. The dorsal zone is 

 divided by an angle in the interior and by the point of entrance of the nerve-roots 

 on the exterior from the ventral zone, V.Z. On their dorsal side the dorsal zones 

 thin out and pass over gradually into the ependyma. The ependyma consists of a 

 single layer of cells. In the dorsal zone the differentiation of the three primary 

 layers of the medullary wall has scarcely begun, but in the ventral zones the three 

 layers are already distinguishable, though not far advanced in their differentiation. 



EC. mes. Epen. >. 



A. has. 



FIG. 187. PIG, 9.0 MM. FRONTAL SERIES 54, SECTION 459. 



/ .has, Basilar artery. Ao.d, Descending aorta of the left side. Ao.^, 7 Third aortic arch. Ao.^, Fourth aortic 

 arch arising from the median ventral aorta. Card, Anterior cardinal vein. cl.II.ex, External portion of the 

 second gill-cleft. cl.III, Third gill-cleft. D.Z, Dorsal zone of the medulla oblongata. EC, Ectoderm . 

 Epen, Ependymal roof of the hind-brain. Hy, Hyoid arch, mes, Mesenchyma. Ot, Posterior wall of the 

 otocyst. P.Ao, Pulmonary aorta. Ph, Pharynx. Som, Somatopleure. V.Z, Ventral zone. X 22 diams. 



In the floor-plate there are two layers. Below the medullary tube lies the ba"silar 

 artery, 14.. has, and below that, not far from the upper wall of the pharynx, lies 

 the small round notochord in the midst of loose mesenchymal cells, which have 

 not yet begun to condense themselves about the notochord. The pharynx is a wide 

 space of rather stnall dorso- ventral diameter, and having a much thinner layer of 

 v.KK 1 ., ,11 on its dorsal than on its ventral side. Above the pharynx on either 

 side lies the section of the descending aorta, Ao.d. The reference line to this 

 vessel crosses a dark mass of cells which belong to the ganglion nodosum of. 4 he 

 tenth nerve. Below the pharynx the section shows the third aortio arch, AO.T,, 

 and the fourth aortic arch, Ao.^, just springing off from the median aortic /trunk 

 above the heart, so that the two fourth arches are connected across the tfiedian 



